Conventionally, Snxe2x80x94Pb alloy solders have made it possible to perform temperature-hierarchical bonding in which first soldering is performed at a temperature of about 330xc2x0 C. by use of a high-temperature solder such as Pb-rich Pb-5 mass % Sn (hereinafter the indication of xe2x80x9cmass %xe2x80x9d is omitted and only numerals are recited) solders (melting point: 314-310xc2x0 C.) or Pb-10Sn solders (melting point: 302-275xc2x0 C.) and in which second bonding is then performed by use of a low-melting point solder of Sn-37Pb eutectic (melting point: 183xc2x0 C.) without melting the first soldered portions. These solders are soft and rich in deformability and, therefore, they can be used for bonding Si chips etc. apt to be broken to a substrate having a different thermal expansion coefficient and also for structural purposes. This temperature-hierarchical bonding is mainly adopted in semiconductor devices in which chips are die-bonded and in semiconductor devices in which chips are flip-chip-bonded such as BGA and CSP etc. In other words, this means that a solder used within a semiconductor device and another solder for bonding the semiconductor device itself to a substrate are used for the temperature-hierarchical bonding. On the other hand, the temperature-hierarchical bonding is used also for bonding power modules etc. used at high temperatures.
The present invention relates to a Znxe2x80x94Al alloy solder and a product in which this solder is used. More particularly, the invention relates to a Pb-free solder which makes it possible to perform temperature-hierarchical bonding on the high-temperature side relative to, for example, an Snxe2x80x94Agxe2x80x94Cu alloy Pb-free solder and which is a thermal-fatigue-resistant solder excellent in rolling workability, and a product in which the solder is used, such as, for example, a semiconductor device and a semiconductor module.
At present, Pb-free design is prevailing in all fields.
As Pb-free solders, Snxe2x80x94Ag eutectic solders (melting point: 221xc2x0 C.), Snxe2x80x94Agxe2x80x94Cu eutectic solders (melting point: 221-217xc2x0 C.) and Snxe2x80x94Cu eutectic solders (melting point: 227xc2x0 C.) come to be used. Although it is desirable that soldering temperatures in surface mounting be low in view of the heat resistance of parts, it is necessary to ensure wettability in order to keep the reliability and, for this reason, actual soldering temperatures in the case of the Snxe2x80x94Agxe2x80x94Cu eutectic solders capable of bonding at the lowest temperatures among Pb-free solders are about 235 to 250xc2x0 C. maximum in consideration of temperature variations within a substrate even if a furnace excellent in controlling for even temperature distribution is used. Therefore, solders capable of withstanding this soldering temperature range are required to have a melting point not less than 260xc2x0 C. At present, there is no soft Pb-free solder for a temperature hierarchical bonding on the high-temperature side which solder can be used in combination with these solders of the low temperature side. As a composition which seems most appropriate for the high temperature side, there is a Sn-5Sb alloy solder (melting point: 240-232xc2x0 C.). However, because the solder melts at the high temperature range, they cannot be used in the temperature-hierarchical bonding.
Further, although an Au-20Sn solder (melting point: 280xc2x0 C.) is known as a high-temperature solder, its use is limited to a narrow range because it is a hard material and its cost is high. Especially, in bonding an Si chip to a material having a different coefficient of thermal expansion or in bonding a large-size Si chip, this Au-20Sn solder is not used because there occurs such a fear as the Si chips are broken due to the high hardness of this solder.
In the invention, there are proposed a material, a system and a structure which are suitable for large-area bonding, for example, Si die-bonding and power module bonding. In the large-area bonding are required soft solder joints which have a thermal fatigue-resisting property and which are free from voids. In addition, it is also required that flux-less bonding be possible when forming these joints.
The object of the invention is to provide a new solder by improving and modifying Znxe2x80x94Al alloy solders which new solder is used as the solder of the high-temperature side. This solder is intended to be used not only in the field of electronic devices, but also in the general structural field.
Representative features of the invention for achieving the object are recited below.
According to the first aspect of the invention, there is provided an electronic device comprising at least one electronic part and a substrate on which the electronic part is mounted, the electronic part and the substrate being bonded to each other by joints comprising Al particles and an Alxe2x80x94Mgxe2x80x94Gexe2x80x94Zn alloy, and the Al particles being connected to each other by the Alxe2x80x94Mgxe2x80x94Gexe2x80x94Zn alloy.
According to the second aspect of the invention, there is provided a semiconductor device in which each of pads of the semiconductor chip and each of leads of a substrate on which the semiconductor chip is mounted are electrically connected by a gold wire, the semiconductor chip and the substrate being bonded by solder joints each comprising Al particles and an Alxe2x80x94Mgxe2x80x94Gexe2x80x94Zn alloy.
According to the third aspect of the invention, there is provided a semiconductor device in which each of pads of the semiconductor chip and each of leads of a substrate on which the semiconductor chip is mounted are electrically connected by a gold wire, the semiconductor chip and the substrate being bonded by solder joints each comprising Cu particles and an Alxe2x80x94Mgxe2x80x94Gexe2x80x94Zn alloy.
In the semiconductor device, the surfaces of the Al particles may be plated with at least one kind selected from the group consisting of Ni, Cu, Ag, Sn and Au.
Also, in the semiconductor device, the surfaces of the Cu particles may plated with at least one kind selected from the group consisting of Ni, Cu, Ag, Sn and Au.
In the semiconductor device, each of the solder joints may include plastic particles.
In the semiconductor device, the material for the plastic particles may be made of at least one resin selected from the group consisting of polyimide, heat-resistant epoxy, silicone, various types of polymer beads, modified types of these materials and a mixture of these.
Further, in the semiconductor device, each of the solder joints may have Sn and/or In particles in addition to the Al particles. This results in a reduction in the coefficient of thermal expansion of a solder.
Further, in the semiconductor device, each of the solder joints may have Sn and/or In particles in addition to the Cu particles. This results in a reduction in the coefficient of thermal expansion of a solder.
In the semiconductor device, each of the solder joints may include particles of at least one kind selected from the group consisting of invar, silica, alumina, AlN and SiC in addition to the Al particles. AlN is aluminum nitride which is known as a high-thermal conductivity ceramic. SiC is silicon carbide which is known similarly as a high-thermal conductivity ceramic.
Also, in the semiconductor device, each of the solder joints may include particles of at least one kind selected from the group consisting of invar, silica, alumina, AlN and SiC in addition to the Cu particles. This results in a reduction in the coefficient of thermal expansion of the solder.
In the semiconductor device, the Alxe2x80x94Mgxe2x80x94Gexe2x80x94Zn alloy may consist, by mass, of 3 to 7% Al, 0.5 to 6% Mg, 1 to 5% Ge, and the balance Zn and incidental impurities.
In the semiconductor device, the Alxe2x80x94Mgxe2x80x94Gexe2x80x94Zn alloy may include a 4Al-3Mg-4Gexe2x80x94Zn alloy.
According to the fourth aspect of the invention, there is provided an electronic device such as, for example, a semiconductor module or a multi-chip module, in which the semiconductor device is mounted on another circuit board by use of a Pb-free solder (, that is, a solder which does not positively contain any lead) such as an Snxe2x80x94Agxe2x80x94Cu alloy solder and etc. In this case, temperature-hierarchical (, that is, high-temperature and low-temperature) bonding can be realized for the Pb-free solder bonding of the semiconductor device (package) and for the mounting of the semiconductor device on another substrate.
According to the fifth aspect of the invention, there is provided a method of producing a semiconductor device, comprising the steps of: dicing a semiconductor wafer; mounting on a lead frame a chip cut from the semiconductor wafer (die-bonding); wire-bonding the semiconductor chip and the lead frame together by means of a gold wire etc.; resin-molding the semiconductor chip; and cutting the lead frame, wherein the semiconductor chip is mounted on the lead frame by use of a solder comprising Al particles and an Alxe2x80x94Mgxe2x80x94Gexe2x80x94Zn alloy, and the Al particles are connected to each other by the Alxe2x80x94Mgxe2x80x94Gexe2x80x94Zn alloy.
According to the sixth aspect of the invention, there is provided a method of producing a semiconductor device, comprising the steps of: dicing a semiconductor wafer; mounting on a lead frame a chip cut from the semiconductor wafer (die-bonding); wire-bonding the semiconductor chip and the lead frame together by means of a gold wire etc.; resin-molding the semiconductor chip; and cutting the lead frame, wherein the semiconductor chip is mounted on the lead frame by use of a solder comprising Cu particles or Cu particles subjected to a surface treatment such as Ni/Au plating and an Alxe2x80x94Mgxe2x80x94Gexe2x80x94Zn alloy, and the Cu particles or surface-treated Cu particles are connected to each other by the Alxe2x80x94Mgxe2x80x94Gexe2x80x94Zn alloy.
In the die bonding step of the semiconductor device manufacturing method, the die-bonding may be performed while jetting a nitrogen gas.
Also, the die-bonding step of the semiconductor device manufacturing method may be performed in an inactive atmosphere of a nitrogen gas etc. or in a reducing atmosphere of a hydrogen gas etc. or in a mixed atmosphere of both of them.
According to the seventh aspect of the invention, there is provided a semiconductor module or multi-chip module manufacturing method having the steps of mounting the semiconductor device on another substrate by use of a Pb-free solder such as an Snxe2x80x94Agxe2x80x94Cu alloy solder.
In the temperature-hierarchical bonding, it is thought that, in a case where, even if a part of the high-temperature side solder having been already bonded is re-melted, most of the high temperature side solder do not remelt, the high-temperature side solder can, in most cases, withstand the process during the solder bonding for the post-bonding operation. That is, it is necessary that the high-temperature side solder which has already been bonded have a bonding strength capable of withstanding the reflow condition of 260xc2x0 C. Accordingly, the present inventors have remarked Znxe2x80x94Al alloy solders of a low cost as predominant high-temperature solders and have researched them for improving and reforming these solders. Some high-temperature solders used in power modules must have a melting point not less than 250xc2x0 C. and are, at the same time, required to meet such a severe condition as even a part of a solder must not melt, insofar as severe, some product fields are concerned. For this reason, the inventors have produced solders which do not melt even at temperatures not less than 260xc2x0 C. and other solder which have a strength even at 260xc2x0 C. even when they partly melt at this temperature.
Because the Znxe2x80x94Al alloy solders have many shortcomings, it is necessary to remove the shortcomings. A Zn-5Al eutectic solder has a high melting point of 382xc2x0 C. and hence it is especially difficult to perform soldering at a relatively low temperature of about 300xc2x0 C. In this case, it is necessary for the solder to have a composition which maintains a strength at a temperature not less than 260xc2x0 C. even when a part of the solder melts. Main requirements and problems are explained below and means for the solution thereof are described.
First, (1) the melting points of Znxe2x80x94Al alloy solders must be lowered to a level of 300xc2x0 C. Next, (2) in a case where the Znxe2x80x94Al alloy solders are used in a foil form, the Znxe2x80x94Al alloy solders must be rolling-workable. Furthermore, in view of the respect that the bonding of different kinds of materials is often performed, the following requisites are needed for a temperature cycle test etc.: (3) solder joints must have thermal fatigue-resisting properties; (4) the solder itself must have a flexibility; (5) in the state of a foil, the Znxe2x80x94Al alloy solders must be free from voids; (6) it is possible to prevent the solders from being oxidized (soldering in a N2 gas); and (7) the Znxe2x80x94Al alloy solders must be excellent in corrosion resistance. Solders that can meet these requisites are suitable for the temperature-hierarchical bonding. Incidentally, it is not always necessary to meet all of these requisites.
To meet the requirements (1) and (2), the inventors has researched a Znxe2x80x94Al alloy solder composition which has a melting point at a lowered level of 300xc2x0 C. and which is excellent in rolling-workability. Although a Zn-5AL solder is a eutectic solder having a melting point of 382xc2x0 C., its melting point is too high and the solder poses the problem of oxidation. By adding both Mg and Ge to a Znxe2x80x94Al alloy solder, the melting point can be lowered by a certain level and, at the same time, it is possible to ensure the rolling-workability and to improve the oxidation resistance. Mg is effective in preventing the grain-boundary corrosion and lowers the melting point, however, the Mg adding of an excessive amount makes the material brittle. The rolling working of a Zn-5Alxe2x80x94Mg alloy is impossible. However, the present inventors have found that the rolling working becomes possible when Ge is added to the Zn-5Alxe2x80x94Mg alloy. Namely, Ge is an element necessary for the rolling working. Although the workability of the Zn-5Alxe2x80x94Ge alloy itself is good, there are such problems as the melting point is too high and as the grain-boundary corrosion occurs. Accordingly, it is important to add both Mg and Ge in the Znxe2x80x94Al alloy.
Further, when Sn, In and Ga are added in the Znxe2x80x94Al alloy, the soldering temperature is decreased although the liquidus line temperature thereof does not decrease so much, with the result that the soldering at about 330xc2x0 C. becomes possible. The solderability thereof is improved by adding much amounts of Sn and In. However, in a case where a much amount of Sn is added, the melting of an Snxe2x80x94Zn phase (having a solidus line temperature of about 197xc2x0 C.) occurs. Also in the case of the addition of In, the melting of an Inxe2x80x94Zn phase (having a solidus line temperature of about 144xc2x0 C.) occurs. However, even in the case where the partial melting occurs, no problem occurs in many cases because the strength can be maintained at the high temperature by the most of the solder other than the melted portions. Therefore, by dispersing the relatively soft phases each having the low melting point, the bonding can be performed at a temperature very close to 300xc2x0 C. and it become possible to keep a strength at the high temperature (, that is, in the case of an Sn-3Ag-0.5Cu alloy solder, the usual reflow temperature thereof becomes 250xc2x0 C. at maximum).
Next, the improvement in the thermal fatigue property (Paragraph 3) and the flexibility of the solder itself (Paragraph 4) can often be simultaneously achieved. That is, the Znxe2x80x94Alxe2x80x94Mgxe2x80x94Ge alloy itself is hard in hardness and is poor in deformability although the solder has a high strength, a large rigidity and a good workability. Thus, in the case of using this solder for a large Si chip etc., there occurs such a fear as the fracture of the chip occurs. Therefore, by adding much amounts of In and Sn to improve the deformability and by dispersing the soft, pure Al particles of a high melting-point to soften the whole of the solder, it becomes possible to reduce the stress applied to the joints formed by the solder, so that the thermal fatigue properties can be improved. In order to disperse the Al particles in the solder, the aluminum and the Znxe2x80x94Alxe2x80x94Mgxe2x80x94Ge alloy all prepared in the form of particles are mixed and dispersed to thereby prepare a powdered mixture. The mixture is then compacted and sintered under a static pressure in an inactive atmosphere or in a reducing atmosphere and are further rolled to thereby form a foil. By compacting the Znxe2x80x94Alxe2x80x94Mgxe2x80x94Ge particles and Al particles at a high temperature under a static pressure after these particles are mixed at a room temperature in an inactive atmosphere, a resulting structure is such that the Al particles become so-called xe2x80x9cislandsxe2x80x9d and the Znxe2x80x94Alxe2x80x94Mgxe2x80x94Ge particles having a low melting point becomes so-called xe2x80x9cseaxe2x80x9d, that is, the Al particles come to be uniformly dispersed discretely and the space among the Al particles comes to be filled with the Znxe2x80x94Alxe2x80x94Mgxe2x80x94Ge alloy phase which acts to connect the discrete Al particles to each other. In this case, it is necessary that the Znxe2x80x94Alxe2x80x94Mgxe2x80x94Ge particles related to bonding be the xe2x80x9csea.xe2x80x9d By providing such a state as the soft Al is uniformly dispersed in the solder, it becomes possible to relieve thermal impacts and to reduce the applied stress, whereby the thermal fatigue resisting property can be improved. Further, in order to strengthen the bonding between each of the Al particles and the Znxe2x80x94Alxe2x80x94Mgxe2x80x94Ge alloy solder, the surface of the Al particles may be plated with a thin coating of Ni/Au plating (, in which the Ni and Au layers are made to have such a thin thickness as to be 0.1 to 0.5 xcexcm and 0.1 xcexcm, respectively, and in this case the Ni may be diffused in the solder, that is, a thick layer of Ni deteriorates the deformability of the Al,) or with a thin coating of Ag plating etc.
Further, in order to soften the solder, fine plastic particles plated with Au may be dispersed in the solder. Also, by dispersing and mixing low-thermal-expansion particles which are metallized with a metal wettable through the solder or which are plated with an Sn alloy solder on the metallization layer, it is possible to relieve the stress and to thereby improve the thermal fatigue resisting property of the joints formed by the solder.
In order to remove the voids (Paragraph 5) from the foil, it is effective to perform the sintering in a vacuum. However, even in a case of performing no vacuum treatment, it is possible to suppress the voids to a degree of not more than 3%.
As regards the preventing of the oxidation (Paragraph 6), it is realized by performing in a N2 gas atmosphere the die-bonding insofar as the Znxe2x80x94Al-MG-Ge alloy solder is concerned.
Further, regarding the corrosion resistance (Paragraph 7), the solder of the invention can clear high-temperature high-humidity test conditions (85xc2x0 C., 85% RH, 1000 h) insofar as the Znxe2x80x94Al-MG-Ge alloy solder is concerned.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.